The measurement of total carbon dioxide concentration. (TCO.sub.2) in serum or plasma is a routine analysis performed in clinical laboratories as a patient profile test. In particular, measurement of TCO.sub.2 is useful in determining the acid-base status of a patient. For example, plasma TCO.sub.2 can be used by a physician in the diagnosis of a patient in renal failure or acute acidosis.
TCO.sub.2 in serum or plasma is comprised of three major chemical forms: dissolved CO.sub.2 (3%); carbamino (R--NHCOO.sup.-) derivatives of plasma protein (33%); and bicarbonate (HCO.sub.3.sup.-) anion (64%). Other minor forms include carbonic acid (H.sub.2 CO.sub.3) and carbonate anion (CO.sub.3.sup.=).
Most of the procedures that quantify TCO.sub.2 in serum or plasma involve acidification of the sample to convert all of the CO.sub.2 -containing species to CO.sub.2 gas, as is summarized in equation (1): ##STR1## Once liberated, the CO.sub.2 gas can be measured manometrically in batch analysis or by continuous-flow procedures where the CO.sub.2 gas diffuses across a membrane into a bicarbonate-containing solution thereby lowering the pH of the solution. The change in pH can be detected either potentiometrically using a pH glass electrode or spectrophotometrically if the bicarbonate buffer contains a pH indicating dye. Additionally, there are detection methods that use enzymes or bicarbonate-selective electrodes to quantify TCO.sub.2. These techniques apply to serum or plasma samples. Therefore, centrifugation of the whole blood sample to separate the cells and plasma prior to analysis is required.